Current studies involve effects of endotoxin and insulin on the development of plasma cell tumors in the BALB/c mouse. Nanogram doses of E. coli endotoxin given to the oil-injected BALB/c mouse increase the rate of plasma cell tumorigenesis. Administration of minute amounts of endotoxin to mice before and after inoculation with plasma cell tumor prevents outgrowth of tumor. We are currently investigating the in vitro behavior of endotoxin in cell culture of established plasma cell tumor. cAMP inhibits the growth of plasma cell tumor in culture and administration of insulin reverses this inhibition. Small amounts of insulin increase the transport of cAMP into plasma tumor cells, and the tracer is present within the cell as 5'AMP. cAMP enhances the entry of 3H-thymidine into plasma cell tumor cells in culture, and small amounts of insulin reverse this effect. The discovery of an insulin-cAMP control system in this tumor cell line is the subject of future investigation in this laboratory.